WT5”BZ]The energies and the magnetization of an electron in a metal cylinder in a magnetic field, at high temperature, and in range of quantum size of the cylindrical radius have been obtained by the WKB method. The results show that the smaller the cylindrical radius of the metal is, the stronger the diamagnetism is, and approximately, in the range of 100nm, the diamagnetism reaches its maximum and is stronger 10 4~10 6 times than that of Landau's. Otherwise, we find a vibration of the magnetization of the system when the metal cylindrical radius become...